The quantity \(\dot{\tau }\)  reveals that most of the absorption occurs at \(r=15\) –30 \(R_{\mathrm {J}}\) .
There, the temperatures are \(T\approx 2000\) –1500 K respectively (not shown),
a factor \(\approx 2\)  lower than the shock temperature.
The extinction
comes mainly from the continuum opacity with a small contribution from the resonant hydrogen absorption.
In general, because of the high temperatures in the postshock region [1]},
the emerging line is usually much broader than the thermal broadening
of the incoming gas.
Equation (REF )) shows that
there are only redshifted absorbers;
layers moving towards the planets absorb photons blueshifted to H \(\alpha \)  in the frame of the accreting gas (see the curved grey line in Figure REF ).
Therefore,
resonant absorption (by the incoming \(n=2\)  electrons) can occur only redward of the rest central wavelength,
approximately equal to the centre of the line emerging from the postshock region. This is the yellow region in Figure REF .
